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690                       Compound Locomotive.

tyres are rolled weldless and fit into annular grooves in the wheel
rim, to resist centrifugal force. The front end of the frame is
supported by a trolley or bogie, which permits certain side move-
ment when travelling round curves. H H is the bogie frame, with
stay rods TT, cc the bogie wheels, and GG the axles. A block or
die 43, curved to a radius from 42, is held by the pin D ; and
guides 44, similarly curved and forming part of the bogie frame,
ride upon the die. If 43 were rigid, the bogie would only swivel
round 42, and would only adjust itself to certain curves; but the
freedom of 43 on D permits a further angular movement, and
the virtual centre 42 is therefore variable. The buffers u u limit
the lateral deviation, and springs gg return the bogie frame to
central position.

Laminated springs> h and 13, and helical springs, 45, placed
between frames and wheels, lessen the shock due to inequality of
permanent way, and the necessary vertical sliding is met by
providing special bearings, S, x, and 14, termed axle boxes, for
the bogie, driving, and trailing axles respectively. R, w, and 15
are the guides in which the boxes rise, and z is a wedge to take up
wear in the main box.

A hand brake r i is used in emergency, but the regular work
is performed by the Westinghouse compressed air brake. The
steam pump 51 fills the main air receiver 5, from which auxiliary
receivers—one to each carnage, and one, 46, for the engine—are
further supplied. From 46 pipes are led to the cylinders 4 4,
and the air pressure moving levers 2 2 put the blocks 3 3 on the
tyres. Upon exhausting, 3 3 are released by springs. When
ascending steep gradients, sand is driven between wheel E and
the rail by means of a steam jet from pipe /, the sand passing
from sandboxes jj, down the pipe k. Cylinder cocks 18 act as
relief valves, and are opened after the engine has stood some

Boiler 20, and firebox 21. Very little description is needed
beyond that at p. 335. Girder stays 53 are of cast steel, and 52
are long stay bolts. A firebrick arch 23 deflects the current of
heated gases over the box, and the ash pan 2 has doors or
dampers, both before and behind, for regulating the draught.
The draught is 'induced' by the exhaust steam escaping at the